The present invention relates generally to the field of standing wave linear particle beam accelerators, and more particularly to an energy switch assembly for a standing wave linear particle beam accelerator having two mechanical plungers, which, when radially inserted into two side cells or cavities of the linear particle beam accelerator, increases the output of the linear accelerator at lower energies.
Medical linear particle beam accelerators that operate at two or more energies suffer from the fundamental limitation that the internal structure cannot be optimized for multiple energies. Because the length of the accelerating structure is fixed, in order to produce both high energy and low energy beams, the cavities are operated at maximum electric field gradient in the higher energy mode and lower gradient in the lower energy mode. The lower gradient causes more electrons to be lost as they travel through the structure, resulting in loss of output in the low energy mode.
To reduce this loss in output, linear particle beam accelerators were developed which employ energy switches. Energy switches typically employ one or more plungers which can be inserted into side cavities of the linear accelerator. Such energy switches concentrate the radio frequency (RF) power in the upstream section of the accelerator structure, before the switch location. In this manner, energy switches cause the gradient to be higher in the upstream portion of the accelerator structure so that electrons are accelerated and captured more efficiently, resulting in higher output in the lower energy mode.
Energy switches may employ radial or transverse plunger arrangements. In energy switches employing a radial plunger arrangement, the plunger is inserted into the side cavity along an axis which lies in a plane normal to the electron beam, while in energy switches employing a transverse plunger arrangement, the plunger is inserted into the side cavity along an axis which lies in a plane parallel to the electron beam. Known to the art are energy switches that employ multiple plungers which are positioned within a single side cavity of the linear accelerator. Such energy switches are used to provide the linear accelerator with three or more output levels.
Prior energy switches achieve high dose rates at low energy, but, because of heating, require direct water-cooling of the plunger during operation. Thus, the energy switches are vulnerable to water leaks and require multiple control system interlocks. Additionally, the side cavity in which the plungers are positioned must have a modified, more complex structure than the other side cavities of the accelerator.
Consequently, it would be desirable to provide an energy switch assembly for standing wave linear particle beam accelerators that increases the output of the accelerator at lower energies, but which does not cause excessive heating, and thus, does not require water-cooling. Additionally, it would be desirable to provide an energy switch assembly that does not require the use of side cavities having a special internal geometry.